1. Field of the Invention
This invention relates to a television camera, particularly having a projector for a test pattern chart used to regulate image taking devices, which projector may be fixedly or detachably mounted in the camera.
2. Description of the Prior Arts
Most of the color television cameras which are presently used for broadcast are of such a type that three pickup tubes are used. These television cameras must be adjusted in accordance with a sequence of operations to ensure good quality of colored pictures after an old pickup tube has been replaced by a fresh pickup tube, or during periodical maintenance, or before the cameras are actually utilized. This is normally accomplished mainly by illuminating test patterns for resolution, registration scale, gray scale and the like in a uniform manner, positioning the camera lens properly at a location opposed to the test patterns before they are framed into their normal sizes, and thereafter operating the regulators of a central control unit, part of which is located on the camera. In order to take such test patterns, transparent and reflective test patterns are normally utilized to be illuminated by an external light. This process, however, is troublesome and time-consuming since a pattern must be replaced by another pattern to be in turn set in each image-taking operation.
Modern color television cameras have been remarkably improved in stability. Nevertheless, these cameras must be sometimes checked by using the test patterns even at work. For such a reason, it is difficult to use the above adjusting process.
In order to overcome the above problems for increasing the workability of the color television cameras, there has been proposed a color television camera which is provided with a zoom lens containing a test-pattern projector therein, the zoom lens being normally used as an objective lens in the camera. However, this camera is used only as an auxiliary one because of its imperfect performance.
The prior art will now be described in connection with the accompanying drawings.
FIG. 1 illustrates a process in the prior art wherein test patterns are image-taken by illuminating them with an external beam of light. For example, if a reflective test pattern chart 2 for gray scale or the like is illuminated by a beam of light from a source of light 1, the reflected light from the chart 2 passes through a zoom lens X into a color separation prism system Y, the zoom lens consisting of a zoom portion I and a relay portion II. In the color separation prism Y, the light is separated into the respective color portions in red, green and blue zones each of which is focused on the light-receiving surfaces of image pickup tubes 3R, 3G and 3B. The chart 2 is normally illuminated by the source of light 1 under a standard image-taking condition, that is of a color temperature of 3,000.degree. K. and an illumination intensity of 2,000 luxes. The light-receiving surfaces of the image pickup tubes are adjusted in brightness by an iris diaphragm 4 in the zoom lens X.
On the other hand, the prior art image-taking process which utilizes a television camera with a pattern projector contained therein includes a step of illuminating a transparent test pattern chart 2' as for a gray scale or the like through a condenser lens 9 by a source of light 1' which is a tungsten lamp, for example. The light passed through the chart 2' is reflected by a reflector 12 toward a projecting lens 8. The light from the projecting lens 8 is reflected by the surface 12 of a prism-shaped half mirror 6 which is disposed between a forward group of relay lenses 5 and a rearward group of relay lenses 7 in the relay portion II of the zoom lens X. The reflected light is thereafter separated into the respective color portions in red, green and blue zones each of which is transmitted on the image pickup tubes 3R, 3G and 3B, respectively. At this time, the image of the chart 2' is formed on the light-receiving surfaces of the image pickup tubes 3R, 3G and 3B by the projecting lens 8 and the rearward group of relay lenses 7.
Normally, the light source 1 for the contained pattern projector is energized by a source of electric power which is common to the circuit of the color television camera and which is of a stabilized D. C. voltage of 24 V and a current capacity of 0.75 A. Accordingly, a high color temperature lamp such as a halogen lamp, an optical lamp with a flatwise filament or the like cannot be used as the light source 1' so that the conventional tungsten lamp will be used with its color temperature of 2,550.degree. K..+-.50.degree. K. which is lower by about 450.degree. K. than that of the light source 1 as an external light.
The zoom lens X has a spectral transmittivity that absorbs the light portion in the blue zone more than those in the green and red zones because the zoom lens is generally made of glass having high refractive index in consideration of correction in aberration. On the other hand, the optical system for the pattern projector has a total thickness much less than that of the zoom lens and is made of a relative small amount of glass having high refractive index so that such an optical system will absorb less light in the blue zone compared with the zoom lens X.
If the light-receiving surface of the image pickup tube 3G for light in the green zone has a value in brightness, the light-receiving surface of the pickup tube 3R for light in the red zone will be relatively bright in the image-taking system using the contained pattern projector whereas the light-receiving surface of the pickup tube 3B for light in blue zone will be relatively bright in the image-taking system using the external beam of light. Therefore, the image-taking system using the pattern projector includes a color filter (spectrum-correcting filter) 10 and a neutral density (ND) filter 11 both of which are located near the condenser lens 9 of the contained pattern projector for causing the brightness to coincide with that in the image-taking system using the external beam of light.
In both the prior art image-taking systems, therefore, they are coincident with each other only in a specific color temperature (3,000.degree. K. in the above embodiment).
Recently, there have been developed high performances compact television cameras which can be widely used to relay a broadcast from a stadium, theater or the like other than an indoor studio. Moreover, even news cameramen have utilized television cameras of small size which are called an electro news gatherer (ENG), instead of 16 mm moving picture cameras.
Therefore, the television cameras must image-take (i.e. effect protosensitive conversion of) objects over a very wide range of illumination. For example, if an outdoor object is taken, its color temperature will change moment by moment in such a manner that it is 2,000.degree. K. to 4,000.degree. K. in the morning and evening whereas it is 6,000.degree. K. to 7,000.degree. K. in the day. Under such an illuminating condition, the prior art television cameras were adjusted in the following manner. The conventional television cameras are set at a design-reference color temperature of 3,000.degree. K. or 3,200.degree. K. For example, when an object is image-taken under an illuminating condition of color temperature, 6,000.degree. K., by a television camera having its design-reference color temperature of 3,000.degree. K., a color-temperature changing filter contained in the television camera is used to change the color temperature from 6,000.degree. K. to 3,000.degree. K., which filter is normally disposed between the zoom lens X and the color separation prism system Y, in such a manner that the gray scale chart illuminated at the color temperature of 6,000.degree. K. is image-taken through the zoom lens X to regulate the amplification factors at the outputs of the red and blue pickup tubes to adjust a balance in white and black. If such a color changing filter is not available, any filter having the nearest changing capacity can be used for the same purpose.
In the prior art pattern projector, however, its color temperature is fixed so that the pattern projector cannot provide for the change of color temperature as described hereinbefore.
It is apparent from the foregoing that the prior art pattern projector contained in the television camera cannot provide, for various reasons, the same image-taking state of test pattern as in the image-taking system using the external source of light so that the projector will be used only as an auxiliary instrument.